If you have a question which is applicable to two or more different communities but you don't get an answer in the first community you post it to, is it acceptable to ask it in one or more others? If so, how long would people suggest waiting? 24 hours or so seems fair except in urgent situations...?

Ask the question on the site you think is most applicable. If, like in this case, it does not get any answer, ask a moderator to migrate the question, or alternatively, delete it and re-ask it.

If there's one thing that is disliked, it is duplicating effort across the community. It is considered rude to ask one group of people, then ask another group entirely. Migrating the question allows effort to happen on one site and then be moved to the other site.